We will examine intercellular exchange in crayfish giant axons using techniques such as iontophoretic injection of electron dense tracers followed by ultrastructural observations of the location of the marked substances. From such studies, we expect to determine the cellular mechanisms by which proteins and other substances are mutually exchanged between crayfish giant axons or adjacent glia and whether such substances may be trophically important to the survival of severed giant axons. We expect that, compared to vertebrates, it will be easier to demonstrate the nature and functional significance of trophic exchanges of proteins or other substances in these CNS giant axons because of their large size and because of their highly developed exchange mechanisms, particularly when severed. Nevertheless, we expect that vertebrate axons use qualitatively similar cellular mechanisms for protein exchange, and that results from our model system will be directly applicable to vertebrates. A better knowledge of glial/neuronal or neuronal/neuronal trophic interactions is needed in order to develop better ways to treat or cure diseases such as multiple sclerosis or muscular dystrophies which may be caused by pathological changes in trophic relations in neuromuscular tissue.